A computer system in which a plurality of storage apparatuses are formed as a cluster is known from a viewpoint of enhancement of reliability of the system against failures. This computer system is designed to realize dual data storage by connecting the plurality of storage apparatuses.
Specifically speaking, a copy pair relationship is established between a volume of a storage apparatus constituting a first site and a volume of a storage apparatus constituting a second site; and one volume becomes a primary volume and the other volume becomes a secondary volume. At normal times, a host system accesses the primary volume and write data to the primary volume is synchronously copied to the secondary volume, thereby realizing dual data storage.
The host system recognizes the secondary volume as the same volume as the primary volume. So, when the host system cannot access the primary volume due to whatever reason, the host system switches a path to the secondary volume at the second site and continues writing data to the secondary volume. This type of computer system is described in, for example, WO 2010/106579.